1. Field of the Invention
The present invention relates to a current-voltage (CV) conversion circuit for converting a plurality of capacitance values into respective voltages.
2. Description of the Related Art
FIG. 7 shows a conventional CV conversion circuit (refer to FIG. 2 in JP 2001-124807A for example).
In this configuration a conventional CV conversion circuit includes an operational amplifier 1 having a grounded noninverting input terminal, a resistor 3 connected between an output terminal and an inverting input terminal of the operational amplifier 1, and a capacitor 2, whose capacitance value to be detected, having one end connected to the inverting input terminal of the operational amplifier 1 and the other end connected to a signal voltage source 4. The signal voltage source 4 has a signal having certain voltage amplitude V and a certain angular frequency ω. The signal voltage Vs of the signal voltage source 4 is expressed by Eq. (1):Vs=V·sin(ωt)  (1)
Since the inverting input terminal of the operational amplifier 1 is virtually grounded in FIG. 7, an electric potential at the inverting input terminal is equal to a ground electric potential at the noninverting input terminal. Let the ground electric potential be 0 V, and denoting a capacitance value of the capacitor 2 as C, a current I flowing through the capacitor 2 is expressed by Eq. (2):I=jωC·V·sin(ωt)  (2)
A voltage having amplitude Vo expressed by Eq. (3) is thus generated at an output terminal 5 of the operational amplifier 1:Vo=−jωCR·V·sin(ωt)  (3)where R is a resistance value of the resistor 3.
As apparent from Eq. (3), the amplitude Vo of the output voltage of the operational amplifier 1 is proportional to the capacitance value C of the capacitor 2. Hence, the capacitance value C of the capacitor 2 can be measured by measuring the amplitude Vo of the output voltage.
FIGS. 8A, 8B and 8C are waveforms of voltage and current over time in respective portions of the CV conversion circuit. FIG. 8A shows a waveform of the signal voltage of the signal voltage source 4. A sine wave is given in this case. FIG. 8B shows a waveform of the current flowing through the capacitor 2 and the resistor 3, and FIG. 8C shows a voltage waveform appearing at the output terminal 5 of the operational amplifier 1.
When the capacitances of a plurality of capacitors are measured, the same number of the circuits, each shown in FIG. 7, as capacitors to be measured is needed, thereby allowing the plurality of plurality of capacitances to be measured.
The conventional CV conversion circuit involves a problem that the same number of CV conversion circuits as capacitors to be measured are required, and the scale of the circuit hence increases when the plurality of capacitances are measured.